U.S. Pat. No. 4,235,096 for Gas Detection Apparatus and which issued Nov. 25, 1980 discloses a gas detecting device exposed to exhaust gases in which the detecting device is arranged in series with a reference resistor and compares the voltage at the dividing point between the gas detection element and the reference resistor to determine via a plurality of comparators whether the voltage at the voltage dividing point between the gas detection element and the referenced resistor exceeds a predetermined range or not.
U.S. Pat. No. 4,258,563 for Gas Sensor issued Mar. 31, 1981 discloses an arrangement wherein a predetermined voltage is applied across a series circuit including a gas sensing element and a reference resistor to detect the gas composition in an exhaust gas by a voltage at the junction point between the gas sensing element and the reference resistor. Provision is made for monitoring the voltage at the junction point and another circuit responsive to the monitoring circuit for changing the resistance of the reference resistor so that when the voltage at the junction point exceeds a predetermined range, the resistance of the reference resistor is changed to effect a correct reading if the resistance of the gas sensing element changes with operating temperature or with time.